The present invention relates to a differential pressure sensor comprising a housing formed of two portions that are assembled together and that define a closed internal cavity between them, a deformable membrane dividing said internal cavity into two non-communicating chambers which are in communication with sources of fluid under pressure by means of two orifices respectively formed in said portions of the housing, said membrane being made in such a manner as to constitute a plate of first and second variable capacitance capacitors by being associated with two other plates installed in stationary manner in the housing of the sensor on either side of the movable central plate formed by the membrane, and sensor means that is sensitive to the deformation of the membrane as a result of a difference between the pressures existing in the two chambers in order to produce an electrical signal representative of said pressure difference.
Presently, sensors of this type are used in particular to measure very small differences in pressure (less than 1 Pa). From the fact that the sensors operate on the principle of a differential pair of capacitors rather than on the principle of a single capacitor, they are less sensitive to the effects of temperature affecting pressure measurement. Such effects constitute the main difficulty to be resolved in the manufacture of differential pressure sensors. Those that are most often seen in practice are of two types. They are expressed firstly by deformation (e.g. expansion) of the membrane which, even when displacement is very small, causes a variation in xe2x80x9cpressure zeroxe2x80x9d, and secondly by a variation of the sensitivity of the measurement device (measured in V/Pa) if an element of the sensor is deformed (plates, membranes, housing portions, etc. . . . ). It has been observed, for example, that a displacement of the membrane by about 1 xcexcm caused by a temperature variation is interpreted in the measurement as a pressure difference of 1 Pa. The advantage of the above-mentioned sensors is imparted by the symmetry of their structure which enables them to compensate for the effects of temperature, any increase and any decrease in the capacitance of the first variable capacitor respectively causing a corresponding reduction and a corresponding increase in the capacitance of the second variable capacitor.
However, capacitance sensors present a certain number of problems. One of the problems, which relates to the mechanical structure of the sensor, results from the very costly use of insulating material serving firstly to ensure sealing where each plate passes through the corresponding portions of the housing and to maintain the mechanical dimensions over the working temperature range, and secondly to ensure symmetrical setting of the plates, since the slightest variation in the distance between the plates and the membrane distorts measurement.
Another fundamental problem of that type of sensor results from the cabling which connects the sensor to the measurement device and which results in amounts of stray capacitance that are significant relative to the capacitances which vary over time as a function of pressure and of temperature. Many instances of stray capacitance can be observed, in particular due to the wires connecting each of the plates to ground which is referenced on the housing of the sensor, said ground having the drawback of being situated in the proximity of the wires which connect each plate to the measurement device. Stray capacitance is also created between the bushing of each plate and the portion of the sensor housing which is associated therewith. It is easily understood that the existence of all the stray capacitance can only be harmful to the accuracy of the measurement of the pressure difference, since capacitance can vary just as much when the wires move as under the effect of deformation or variation in the characteristics of the elements which make up the sensor.
Finally, yet another problem can be noted that frequently occurs with that type of sensor, the problem being that of guaranteeing that the membrane remains insulated over time so as to prevent dust, corrosion, moisture, etc. from creating potential defects which distort the measurements. In practice, insulating membranes and an intermediate dielectric fluid are often used, the deformable measurement membrane being immersed in the fluid between the insulating membranes. As a result, sensors are difficult to make and their cost is high.
An object of the present invention is thus to provide a pressure sensor which is simple to manufacture, low cost, capable of measuring very small differential pressures, e.g. of about 1 Pa or less, while being much less sensitive to variations in temperature and in the surroundings in which it operates.
Another object of the present invention is to provide a pressure sensor which is relatively insensitive to the stray capacitance of its components.
To this end, in the differential pressure sensor of the invention the sensor means comprise a measurement circuit for measuring the corresponding capacitances of the first and second variable capacitance capacitors. The circuit is made up of first and second double-sided interconnected circuits, of size similar to that of the membrane, which are respectively disposed in the two non-communicating chambers close to the membrane and parallel to said membrane in such a manner as to divide each of the chambers into two communicating portions. The interconnected circuits are fixed to the membrane in its peripheral region by connection means. The faces of the circuits which face the deformable membrane each comprise a region on which the fixed plate of the corresponding first or, second variable capacitance capacitor is installed. The faces of the circuits which face away from the membrane each comprise a respective region on which the components that are sensitive to the displacement of the membrane towards one or other of the fixed plates are respectively mounted, and a respective region that is insensitive to stray capacitance, which regions are respectively constituted by the peripheral regions of the circuits, another region that is insensitive to stray capacitance being provided on one of said faces facing away from the membrane for mounting the components which form the remainder of the measurement circuit.